Water Saturation Effects on Cyclic Gas Injection in Shale Oil Reservoirs

Abstract

Summary A substantial portion of fracturing water is retained in shale formations, raising near-fracture water saturation, reducing hydrocarbon relative permeability, and potentially weakening subsequent cyclic gas-injection performance. This behavior is further complicated by shale's multiscale pore structure and wettability differences across fractures, macropores, and nanopores. We develop a three-phase, multicomponent, multiscale transport model for shale. Shale is represented by multiple continua of natural fractures, macropores, and nanopores with wettability contrasts: fractures and macropores are water-wet, whereas nanopores are oil-wet. Capillary-pressure induced phase-pressure differences drive inter-continuum water exchange, while hydrocarbon exchange is governed by chemical-potential gradients that drive species advection and diffusion. The results show that capillary-driven water redistribution in the macropores is a key factor controlling primary production and CO2 cyclic injection or huff-n-puff (HnP) performance. During primary production, this redistribution benefits oil transport in larger pores but restricts it in smaller pores. During CO2 HnP, however, water retention reduces recovery across all pore sizes. Water retention limits oil and gas transport, particularly in smaller pores and nanopore-dominated systems. Increasing the macropore fraction provides more conductive pathways and reduces water blockage during cyclic gas injection; however, neglecting capillary pressure can overestimate oil recovery. Matrix water has a much stronger effect on recovery than fracture water, while nanopore wettability has only a minor influence. This study investigates the effects of capillary pressure and water saturation on primary production and subsequent cyclic gas-injection performance, guiding HnP design.